Articles requiring a high strength to weight ratio, such as aircraft structural parts, are increasingly being manufactured by surfacing a rigid structural foam core with a hard smooth skin material. There have been many prior art attempts to obtain rigid structural foam cores having a hard smooth skin material thereon. Bauer in U.S. Pat. No. 3,906,137, and Brand et al in U.S. Pat. No. 4,335,182, produce such composite bodies by heating structural foam above its softening temperature. Brand et al compressed the core to predensify it and then heat it to a foaming temperature for expansion. This requires the use of an autoclave or other external pressure source. Smith in U.S. Pat. No. 3,873,654, Reichert in U.S. Pat. No. 4,292,101 and the Bauer patent teach composite body consolidation methods that require application of compression pressure directly to the core.
Other prior art methods include those of Engelke et al in U.S. Pat. No. 4,521,354, McCarthy in U.S. Pat. No. 4,471,020, Carter in U.S. Pat. No. 4,321,825 and Doerfling in U.S. Pat. No. 4,379,103 who use cast-in-place, unpolymerized (uncured) foam materials in a sandwich structure.
In U.S. Pat. 4,187,353 to Schroeder, the inventor overlays the top and bottom surfaces of a polyacrylimide-polymethacrylimide foam block with a polyester prepreg and then heats the structure to 145.degree. C. in a hydraulic press under 1.5N/mm.sup.2 (218 psi) pressure for three minutes. The structure was removed from the press in a hot state. It had changed shape to the shape of the cavity of the tool since as noted by Schroeder "the shaped piece . . . (was) free of portions assuming the shape of the original form . . . and is of a thickness which corresponds to the cavity of the tool". In this process the geometry of the formed article obviously changes from that of the starting foam block. In "rigid" foams, if the structural integrity of the material is to be maintained, geometry changes can only be effected above the softening temperature of the material. In his examples, this inventor's structures are formed in a press utilizing the application of external pressure. The pressures utilized by Schroeder were typically much higher that those commonly utilized in the aircraft industry. In the aircraft industry, the maximum pressure typically utilized is 100 psi and this is normally only attainable in conjunction with the use of an autoclave.
In a further U.S. patent, U.S. Pat. No. 4,292,101 to Reichert, the inventor describes a process that uses an expanding closed cell "depressible" core of a polyether, a silicone rubber, a phenolic or an epoxy material. This core is overlayed with a lay-up of skin material that will form the product being molded. The core and the skin material are then compressed into a mold under "an initial prestress compression force" as the mold is closed. The mold is heated to "expand the gases captured within the cellular core . . . causing it to exert hydraulic forces . . . " Such expansion of the core by the gases trapped therein is achieved by utilizing as core materials, materials that in the words of the inventor "plasticizes sufficiently when heated". In this process it is evident that the core materials are raised to a temperature above their softening temperature, i.e. their thermoelastic temperature, for such plasticizing to take place.